Various methods have been proposed in the past for producing a ceramic porous body having communication macropores. Some of these methods involve (1) partial binding of a ceramic material, such as a method of coating ceramic particles with a binder, molding and then sintering them together with pores remaining between the particles (Patent Document 1) and a method of joining particles by sintering the contact points (Patent Document 2). Methods involving (2) removal of components in the ceramic include a method of creating pores by heating and breaking down a premixed component so that it vaporizes and is expelled outside the system (Patent Document 3), and a method of creating pores by using chemical treatment to dissolve a specific component in a composite solid (Patent Document 4).
However, in these methods the strength is said to be poor because it is difficult to control the diameter and distribution of the pores, and difficult to produce a porous body with high porosity. There are also (3) methods using gas, such as a method in which a ceramic slurry is agitated and foamed with a foaming agent (Patent Document 5) and a method in which a ceramic slurry is mixed with hollow spheres that are then removed by baking (Patent Document 6). Extremely high porosity can be obtained by using these methods to produce continuous pores, but high strength is still difficult to obtain.
Other methods use (4) soft urethane foam with a continuous bubble structure, including a method in which urethane foam is impregnated with a ceramic slip and burnt to remove the soft urethane (Patent Document 7). In this method there is a risk that cracks will occur in the ceramic framework due to the large amounts of gas produced.
Methods using (5) ice have also been proposed, such as a method of producing a ceramic porous body by freezing water. That is, in this method a ceramic porous body having a highly oriented composite porous structure is produced by cooling a water-based slurry in a mold from the bottom of the mold to thereby grow ice crystals in a single direction, and sublimating the water by freeze-drying (Patent Document 8). Although this method is revolutionary, the pores are oriented in a single axial direction, with little degree of freedom in controlling the pore shape, and since the frozen molded body is composed of powder and ice, it is extremely difficult to handle after freezing and drying if it is a highly porous body, and it is difficult to obtain a highly porous body without cracks.
The inventors in this case have already published a technical paper relating to a Gelate Freezing Method (Non-patent Document 1). According to this paper, it is possible to produce a ceramic porous body with continuous macropores by drying using the gelate freezing method. However, with the technique of this paper, when a slurry with a solids concentration of less than 28 vol % is used in order to obtain a porous body with a porosity of over 72%, cracks occur due to differences in drying contraction between the inside and outside of the molded body during drying, and it is impossible to obtain a highly porous body without cracks.
Patent Document 1: Japanese Patent Application Laid-open No. 2004-129552
Patent Document 2: Japanese Translation of PCT Application No. H11-506806
Patent Document 3: Japanese Patent Application Laid-open No. H11-322465
Patent Document 4: Japanese Patent Application Laid-open No. H10-87378
Patent Document 5: Japanese Patent Application Laid-open No. 2004-201594
Patent Document 6: Japanese Patent Application Laid-open No. H8-59367
Patent Document 7: Japanese Patent Application Laid-open No. H11-322467
Patent Document 8: Japanese Patent Application Laid-open No. 2001-192280
Non-patent Document 1: Journal of the Ceramic Society of Japan, 113, (2005) 712-715
Under these circumstances, and in light of the aforementioned prior art, the authors in this case succeeded, as a result of exhaustive research aimed at developing a novel ceramic porous body production method whereby a ceramic porous body with a porosity of 72% to 99% and continuous pores 10 μm to 300 μm in diameter could be produced by methods that would control the shape of the pores and provide both high porosity and macropores in a strong molded body with excellent handling properties, in applying the aforementioned “Gelate Freezing Method” to high porosity for the first time, and in constructing a new technique that combines “controlled atmospheric substitution-type drying method” by means of vacuum drying, humidity-controlled drying, or immersion in a water-soluble organic solvent and air-drying in the thawing and drying steps to thereby achieve a high porosity rate and produce a porous body with continuous macropores which is even suited to ceramic slurries with a solids concentration of less than 28 vol %.